Instead of losing ice where massive glaciers meet the sea,
Greenland now sends meltwater rushing into the ocean via a vast
network of lakes and rivers, according to several studies. The
results do not mean that
glaciers have stopped their speedy flow, only that surface
melting now exerts a more powerful influence on ice loss,
researchers said.

"We no longer see giant icebergs calving" from glaciers,
releasing ice into the sea, said Lora Koenig, a glaciologist at
the National Snow and Ice Data Center, who led one of the new
studies. "The majority of water is coming from surface melt."
[ Photos:
Under the Greenland Ice Sheet ]

Koenig discovered that lakes in west Greenland now stay liquid
through the frigid winter, as long as an insulating snow blanket
keeps the water warm. These lakes get a head start on melting the
next summer. "Water is not a good thing to have persisting
year-round," Koenig said Dec. 15 at a news conference. "What this
water is really doing is priming the pump [for melting] for the
next season."

The meltwater boosts sea levels, which are projected to rise by 1
to 4 feet (0.3 to 1.2 meters) by 2100, according to the
National Climate Assessment. Water that percolates beneath
the ice sheet can also lubricate the underside of Greenland's
glaciers, speeding up ice flow. But researchers are still
figuring out where all of this new surface meltwater will end up.

"The water is what we have to follow," said Vena Chu, a
hydrologist and graduate student at the University of California,
Los Angeles.

For instance, each summer, a vast network of rivers appears in
Greenland, channeling meltwater off the ice surface. Researchers
said they want to know how much water refreezes in place, how
much ends up under the ice sheet and how much flows out to sea.
By tracking west Greenland's rivers on satellite images, Chu
discovered that the river water all disappears into moulins —
deep cracks that steeply plunge into the ice, she reported at the
meeting.

"Now we need to know if the water gets stuck in there or if it
comes straight out [to the ocean]," Chu said.

The growing flood of surface runoff has also transformed snow
layers that
blanket the ice sheet, researchers reported Dec. 16.
Typically, the top of the ice sheet is blanketed by partially
frozen, old snow called firn, which can suck up summer meltwater
like a sponge. But 12 years of heavy summer melts have
overwhelmed the firn's capacity in southwest Greenland, said Mike
MacFerrin, a glaciologist and graduate student at the University
of Colorado, Boulder.

The waterlogged snow is now frozen solid in many places, with ice
more than 15 feet (4.6 m) thick just below the surface, he said.
Now, summer meltwater streams over the ice instead of sinking
into the snow. In 2012, this triggered record flooding during a
huge melt event in Greenland, said MacFerrin, who led the
study.

However, in other regions, Greenland's old snow still stockpiles
huge amounts of water. Rick Forster, a glaciologist at the
University of Utah, has uncovered additional evidence of a
shallow aquifer of liquid water in southern and western
Greenland. In 2013, Forster reported that parts of Greenland's
snow firn hold an estimated
100 billion gallons of water through the winter months in the
southeast.

From sea to surface

Koenig said global warming has triggered the shift to surface
melting, which took place between 2006 and 2009. Temperatures in
the Arctic are
rising twice as quickly than at lower latitudes, according to
the National Oceanic and Atmospheric Administration's annual
"Arctic Report Card."

Greenland's glaciers have responded quickly to changing
temperatures in the past, said Anders Bjork, a researcher at the
Natural History Museum of Denmark.

Using historical photographs from Danish aerial surveys of
Greenland, Bjork mapped out the advance and retreat of glaciers
that occurred when temperatures climbed between the years 1900
and 1930. The retreat was more rapid than has been seen in the
last 15 years, he said.

Though the past century of change appears remarkably rapid,
overall, the Greenland Ice Sheet is more resilient than most
people assume, said glaciologist Dorthe Dahl-Jensen, head of the
University of Copenhagen's Center for Ice and Climate. The ice
has survived
900,000 years of climate change, and it would take a
temperature rise of 18 degrees Fahrenheit (10 degrees Celsius)
before a forest starts to grow again in Greenland, she reported
here Dec. 17.

"We are just seeing the beginning of a reaction to the warming,"
Dahl-Jensen said.